A MBS is a point-to-multipoint service by which data is transmitted from one source to a plurality of receivers. For efficient use of radio resources, the same data is transmitted to a plurality of receivers via a common radio channel.
Typically, there are two types of MBS in a general wireless access system, single-Base Station (BS) access and multi-BS access. In the single-BS access, one BS services in one cell, whereas in the multi-BS access, a plurality of BSs forms one MBS zone and provides an MBS service in the MBS zone. Since a plurality of cells within the MBS zone receives the same bursts from an MBS server using an MBS zone Identifier (ID) and provides the MBS service, service continuity and macro-diversity gain can be achieved without handover in case of movement between cells including BSs.
FIG. 1 illustrates an exemplary MBS reference model for a broadband wireless access system.
Referring to FIG. 1, an MBS network includes an MBS media server, an MBS distribution server, one or more BSs, and one or more Mobile Stations (MSs). The MBS media server provides MBS data to the BSs and performs authentication and encryption key distribution for the MSs with respect to MBS contents. The MBS distribution server is responsible for scheduling the MBS data to the BSs. Optionally, the MBS distribution server is not provided and instead, an MBS content server may take over the function of MBS data scheduling. The BS transmits the MBS data received over a backbone network to the MSs via radio interfaces. And, the MS receives MBS data from the BS.
In the broadband wireless access system, a MBS may have the following characteristics.
1) Power consumption minimization: an MS can minimize power consumption during receiving MBS data irrespective of its current operation mode (e.g. normal mode, sleep mode, and idle mode).
2) Mobility: an MBS connection may be seamless for a MS even when the MS moves from one BS to another BS.
3) MBS zone: MBS contents are transmitted through a geographically defined MBS zone and MBS configuration information (e.g. MBS connection ID, encryption key, service ID, etc.) may differ in different MBS zones.
4) Security: MBS contents are transmitted only to authenticated users. The encryption key of Medium Access Control (MAC) Packet Data Units (PDUs) of MBS data is commonly applied to the BSs of an MBS zone.
Hereinafter a description will be made of an MBS zone.
Different MBS parameters (e.g. security key, multicast connection ID, etc.) may be set for different zones and broadcasting of MBS contents may be confined to a predetermined zone. Therefore, when an MS moves to another BS or performs handover during receiving MBS contents, the MS should determine whether its stored MBS information is still valid and whether the MS can receive the MBS contents continuously.
If a serving BS transmits an MBS service to the MS according to MBS parameters different from the MBS information preserved in the MS or the serving BS does not transmit MBS contents to the MS, the MS should access a new BS to update its MBS parameters. To avert this problem, the broadband wireless access system defines MBS zones each including a group of one or more MBS BSs.
The BSs within the same MBS zone transmit MBS contents to MSs using the same MBS parameters. Also, an MBS zone ID is transmitted to the MSs so that the MSs can identify the MBS zone. The MSs can immediately check whether their preserved MBS parameters are valid using the MBS zone ID received from the BSs.
When an MS moves to another BS within the same MBS zone, it does not need to re-configure MBS parameters to receive MBS data. As the BSs of the same MBS zone transmit MBS data in the same radio resources at the same time, the resulting macro-diversity effect may increase the MB reception efficiency of the MSs.
An operation for minimizing the power consumption of an MS that is receiving MBS data will be described below.
The MS can reduce power loss during receiving MBS data irrespective of its current operation mode (e.g. normal mode, sleep mode, and idle mode).
Typically, DownLink-MAP Information Elements (DL-MAP IEs) included in a DL-MAP message are defined to indicate bursts transmitted in a current frame. To receive a broadcast burst, the MS should receive and interpret the DL-MAP message in every frame. In this case, power consumption cannot be reduced.
In contrast, an MBS MAP Information Element (MBS_MAP IE) notifies the MS of a frame carrying an MBS data burst, thereby obviating the need for the MS to decode DL frames each having a DL-MAP message but not carrying the MBS data. Consequently, the power consumption of the MS may be minimized. The power saving effect of the MBS_MAP IE is great especially to idle-mode and sleep-mode MSs. Scheduling information about MBS data bursts may be delivered in the MBS MAP IE that is one of DL-MAP IEs, or by a MAC management message such as an MBS MAP message.
A Multi-cell Multicast Broadcast Single Frequency Network (MBSFN) is a service in which a plurality of BSs transmits the same data at the same frequency, forming an MBS zone.
Conventionally, an MBSFN system carries out no uplink feedback and provides services in a fixed Modulation Coding Scheme (MCS). Hence, a BS provides an MBS service with a constant quality to MSs with no regard to channel status and even MSs in good channel status receive excessive-quality services all the time.
In the broadband wireless access system, an MS and a BS establish a connection for a service flow during a service flow creation procedure for data transmission. At this time, the MS and the BS negotiate service quality parameters and negotiate whether Automatic Repeat reQuest (ARQ) will apply to the connection. When a transmission protocol is applied, the MS and the BS exchange transmission parameters with each other.
However, the broadband wireless access system does not allow retransmission for an MBS connection. Therefore, even if the channel environment gets poor and thus the MS fails to receive some MBS data from the BS or received MBS data has errors, there is no way to correct it.
ARQ is defined only for unicast connection between the MS and the BS in the broadband wireless access system. Because a burst profile (Forward Error Correction (FEC) coding type and modulation type) can be configured based on the downlink and uplink signal quality statuses between the BS and the MS, adaptively according to channel status, the reception probability of retransmission may be increased.
However, MBS data is not unicast data transmitted from the BS to the MS. Considering that the MBS data is broadcast to MSs covered by the BS, it is difficult to provide an appropriate burst profile to all MSs that receive the MBS data.
Although the conventional MBSFN system does not perform uplink feedback for MBS and provides services in a fixed MCS, MBS feedback is required to achieve a higher MBS system performance and Adaptive Modulation and Coding (AMC) based on channel status is also needed.
Feedback based on conventional power control and random power ramping technique makes it difficult to accurately measure the reception power level of a detected common Random Access CHannel (RACH) preamble because of the influence of geometry and multi-path. Accordingly, the number of received MBS feedbacks may not be accurately calculated.
Under a wireless mobile communication environment, channels between the MS and the BS may vary in view of various factors including the relative positions of the BS and the MS and geometrical characteristics that the channels experience. Overall system performance as well as the throughput of the particular MS may be increased by appropriately controlling a modulation scheme, a coding scheme, transmit power, etc. according to the channel characteristics between the MS and the BS. In this context, the channel characteristics need to be known and thus the MS is required to measure the channel characteristics and feed back them to the BS.
Nonetheless, there is no MBS uplink feedback and services are provided in a fixed MCS in the conventional MBSFN (Institute of Electrical and Electronics Engineers (IEEE) 802.16e). Meanwhile, the need for an MBS uplink channel is pressing and the PHYsical layer (PHY) structure, feedback type, detection method, etc. of the MBS uplink channel are to be defined in an IEEE 802.16m system.